Hi Mark,
You will have to excuse my friends Chris and Daz, they haven't been the same since they lost their 'special'
helmets that stop them licking the windows on the bus.
I don't know for sure but I suspect that when the lower buffer board fails, the faulty buffer board is corrupting or distorting the Drive waveform, hence when you disconnect the buffer board, you are then removing the corrupting influence and the drive waveform is then 'clean' and you then get a 'clear' picture at the top of the screen.
If any Techno wizards no any different i'm sure they will chip in
Brian